Continuous roll-to-roll fabrication of cellulose nanocrystal (cnc) coatings

ABSTRACT

A method of large-scale continuous roll-to-roll fabrication of cellulose nanocrystal (CNC) coatings with controlled anisotropy, and CNC-coated flexible substrates prepared thereby. An order parameter of 0.78 is observed in CNC-poly(vinyl alcohol) (CNC-PVA) coating systems at 70% CNC loadings.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation patent application of co-pending U.S.patent application Ser. No. 17/517,354 filed Nov. 2, 2021, which is adivision patent application of U.S. patent application Ser. No.16/643,034 filed Feb. 28, 2020, and issuing as U.S. Pat. No. 11,174,404on Nov. 16, 2021, which claims priority to International PatentApplication No. PCT/US18/49312 filed Sep. 4, 2018, which claims priorityto U.S. Provisional Application Ser. No. 62/555,084 filed Sep. 7, 2017.The contents of these prior patent documents are incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under CMMI-1449358 and1144843 awarded by the United States National Science Foundation. Thegovernment has certain rights in the invention.

BACKGROUND

The present application relates to methods of continuous roll-to-rollfabrication of cellulose nanocrystal (CNC) coatings with controlledanisotropy, and the cellulose nanocrystal (CNC) coated flexiblesubstrate prepared with the methods.

This section introduces aspects that may help facilitate a betterunderstanding of the disclosure. Accordingly, these statements are to beread in this light and are not to be understood as admissions about whatis or is not prior art.

Cellulose nanocrystals (CNCs), also known as nanocrystalline cellulose(NCC), are an alternative renewable raw material derived from abundantresources: wood, plants, algae, tunicate, bacteria, etc. CNCs aredistinguished from other cellulose nanomaterials in that they arerod-shaped and rigid with lengths of 50-1000 nm, widths of 1-20 nm, andhighly crystalline. CNCs have excellent properties, such asnon-toxicity, biodegradability, high specific strength, high thermalconductivity, and optical transparency.

Based on these remarkable properties, CNCs are applicable as areinforcement component in nanocomposites, transparent media in organicelectronics, anti-counterfeiting in security applications, and barriersin packaging applications. CNCs can form a coating, film, aerogel orfoam depending on the desired final application.

The mechanical, thermal and optical properties of CNC materials dependsignificantly on the structural arrangement of the crystalline domain.Depending on the crystal domain organization, CNC materials can beisotropic or anisotropic (isotropic is directionally independent, andanisotropic is directionally dependent). An anisotropic configurationcan exhibit enhanced properties in the orientation direction. Forexample, a four-fold higher thermal conductivity in a specific directioncompared to the isotropic configuration.

Solution casting using mechanical shear force is the most commontechnique for anisotropic CNC film fabrication. Solution casting is aconvenient and inexpensive small-scale processing technique, howeverhigh viscosity solutions are primarily used. High concentrations of CNC(and thus high viscosity solutions) allow for limited mobility of theCNCs during processing and therefore, the coating retains shear-inducedanisotropy in its final structure. However, difficulties exist inachieving a uniform homogeneous thickness and the fabrication process istime consuming for such solutions (an hour to several days).Spin-coating overcomes this limitation, but large area processing is achallenge for this method.

There is, therefore, an unmet need for an inexpensive, fast process withlarge-scale continuous fabrication of a CNC layer on a flexiblesubstrate to ensure s substantially uniform CNC coating with controlledanisotropy.

BRIEF SUMMARY

The present disclosure provides continuous roll-to-roll manufacturingprocesses suitable for large scale manufacture for cellulose nanocrystal(CNC) coatings on a flexible substrate.

In one embodiment, the present disclosure provides a method ofcontinuous roll-to-roll coating of a flexible substrate with at leastone cellulose nanocrystal (CNC) layer, wherein the method comprises:

-   -   a) providing a substantially homogeneous aqueous suspension of        CNC, wherein the aqueous suspension comprises 4-20 wt. % of CNC        and 80-96 wt. % of water;    -   b) providing a roll-to-roll coating device that continuously        feeds flexible substrate;    -   c) treating the surface of the flexible substrate to ensure that        the flexible substrate has a surface energy value equal or        higher than the surface tension of the aqueous suspension of        CNC; and    -   d) transferring the aqueous suspension of CNC to the surface of        the flexible substrate to create a CNC wetted region and        continuously passing the wetted region to a drying unit of the        roll-to-roll coating device to provide substantially dried        CNC-coated flexible substrate, wherein the drying temperature is        between 60-100° C.

In one embodiment, the present disclosure provides a CNC-coated flexiblesubstrate prepared by the method of continuous roll-to-roll coating asdisclosed in the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 : Order parameter for different cellulose nanocrystal-poly(vinylalcohol) (CNC-PVA) coating systems.

FIG. 2 : Apparent viscosity for CNC-PVA (12 wt. % solid loading)nanocomposites system with different CNC percentage at 300 s⁻¹ shearrate (line added to aid the eye).

FIG. 3 : Real time birefringence observation for different coatingformulations.

FIG. 4 : WVTR with corresponding order parameter for different CNC-PVAcoating compositions on a PLA substrate.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodiments. Itwill nevertheless be understood that no limitation of the scope of thisdisclosure is thereby intended.

In the present disclosure the term “about” can allow for a degree ofvariability in a value or range, for example, within 10%, within 5%, orwithin 1% of a stated value or of a stated limit of a range.

In the present disclosure the term “substantially” can allow for adegree of variability in a value or range, for example, within 90%,within 95%, or within 99% of a stated value or of a stated limit of arange.

In the present disclosure the term “nanoparticles” refers particles withaverage particle length between 1-1000 nm, 1-900 nm, 1-800 nm, 1-700 nm,1-600 nm, 1-500 nm, 1-400 nm, 1-300 nm, 1-200 nm, 1-100 nm, 1-90 nm,1-80-nm, 1-70 nm, 1-60 nm, or 1-50 nm.

Roll-to-roll (R2R) is a family of manufacturing techniques involvingcontinuous processing of a flexible substrate as it is transferredbetween two moving rolls of material. R2R is an important class ofsubstrate-based manufacturing processes in which additive andsubtractive processes can be used to build structures in a continuousmanner.

R2R is a “process” comprising many technologies that, when combined, canproduce rolls of finished material in an efficient and cost-effectivemanner with the benefits of high production rates and in massquantities. Today, R2R processing is applied in numerous manufacturingfields such as flexible and large-area electronics devices, flexiblesolar panels, printed/flexible thin-film batteries, fibers and textiles,metal foil and sheet manufacturing, medical products, energy products inbuildings, and membranes to name a few.

A non-limiting list of R2R may be but is not limited to vacuumdeposition, gravure, micro-gravure, flexographic printing, flatbed androtary screen printing, imprint or soft lithography, laser ablation,offset printing, inkjet printing, etc.

Very limited work has been done using R2R with cellulose nanomaterials.Previous studies focused on pure CNF and CNF mixtures for thin or thickcoating formations. See Kumar et al. (2016) Roll-to-roll processedcellulose nanofiber coatings, Industrial & Engineering ChemistryResearch, 2016, 55 (12), 3603-3613. However, some additional additivessuch as carboxymethyl cellulose (CMC) is required to achieve theacceptable coating quality for CNF. Even by adding CMC, the coatedflexible substrate in not transparent. Further, CNF coating achieved isnot anisotropic. An anisotropic coating can exhibit enhanced propertiesin the orientation direction. For example, a four-fold higher thermalconductivity in a specific direction compared to the isotropicconfiguration. Due to high viscosity, only up to 2% of CNF suspensioncan be used for the coating process. Such as lower concentration cannotprovide sufficient thickness to ensure sufficient strength and otherdesired features.

Therefore, the is an unmet need to develop a continuous roll-to-rollmethod suitable for preparing large scale CNC-coated flexible substratefor better coating qualities, lower cost, and faster coating speed,wherein the CNC coating has desirable anisotropic characteristics.

A continuous roll-to-roll manufacturing process suitable for large-scalecellulose nanocrystal (CNC) coatings on a flexible substrate has beendisclosed in the present disclosure. It was found surprisingly that thecontrolled anisotropy can be achieved under such process conditions.

For illustration purpose, the processing parameters for a givenmicro-gravure roll-to-roll process that control the coating structureand properties were examined. For a given gravure roll, the CNCconcentration, the surface energy of a flexible substrate to be coatedby CNC, treating temperatures, gravure speed, substrate speed, and inkviscosity were determined to be important parameters that control thecoating quality. After successful fabrication, CNC coating adhesion wasinvestigated with a crosshatch adhesion test. The adhesive strength ofthe CNC coating was correlated with coating thickness, and the maximumcoating strength was observed for the lowest coating thickness. Coatingswere characterized using atomic force microscopy and UV-VISspectroscopy. Finally, the crystalline domain arrangement of coatingswas determined for coatings made from a variety of CNC concentrations,and the effect of viscosity on CNC alignment was explained by variationof shear rate, which was controlled by the micro-gravure rotation.

The roll-to-roll (R2R) manufacturing process is a versatile techniquefor the fabrication of flexible coatings, polymer solar cells, andflexible electronic devices. Both printing and coating are possible in aroll-to-roll system, which can be coupled with slot-die, gravure, spray,inkjet, nanoimprinting, or rotary screen. In an R2R gravure process, inkis continuously transferred from an ink bath to the gravure cylinder. Adoctor blade is placed over the gravure cylinder to remove the excessink and maintain a constant uniform ink thickness. A thermoplastic,flexible polymer film is used as a substrate, which continuously movesbetween the two rolls. A liquid bridge between the gravure cavity andthe web (substrate) that is stretched and sheared with a moving contactline is the basic mechanism of the ink transfer from the gravure to theweb. Typically, the wet liquid film then passes through a drying chamberand dried. The overall coating quality largely depends on the ink'scompatibility with the substrate, gravure speed rate, web speed, dryingunit, and solvent type. Gravure coating has been used to fabricateorganic thin film transistors, light-emitting polymer diodes, organicphotovoltaics, and electrochromic devices.

In one embodiment, the present disclosure provides a method ofcontinuous roll-to-roll coating of a flexible substrate with at leastone cellulose nanocrystal (CNC) layer, wherein the method comprises:

-   -   a) providing a substantially homogeneous aqueous suspension of        CNC, wherein the aqueous suspension comprises 4-20 wt. % of CNC        and 80-96 wt. % of water;    -   b) providing a roll-to-roll coating device that continuously        feeds flexible substrate;    -   c) treating the surface of the flexible substrate to ensure that        the flexible substrate has a surface energy value equal or        higher than the surface tension of the aqueous suspension of        CNC; and    -   d) transferring the aqueous suspension of CNC to the surface of        the flexible substrate to create a CNC wetted region and        continuously passing the wetted region to a drying unit of the        roll-to-roll coating device to provide substantially dried        CNC-coated flexible substrate, wherein the drying temperature is        between 60-100° C.

In one embodiment, the method of the present disclosure provides thatthe roll-to-roll device may be but is not limited to vacuum deposition,gravure, micro-gravure, flexographic printing, flatbed and rotary screenprinting, imprint or soft lithography, laser ablation, offset printing,or inkjet printing,

In one embodiment, the method of the present disclosure provides thatthe roll-to-roll device is a micro-gravure device.

In one embodiment, the method of the present disclosure provides thatthe roll-to-roll device is a micro-gravure device, wherein the gravurespeed is about 4 rpm-200 rpm, 4 rpm-100 rpm, or 4 rpm-75 rpm.

In one embodiment, the method of the present disclosure provides thatthe roll-to-roll device is a micro-gravure device, wherein the rollerspeed is at least 0.2 m/min, 0.5 m/min, 1.0 m/min, 2.0 m/min, or 3.0m/min. In one aspect, the roller speed is about 0.2-20 m/min, 0.2-10m/min, 0.2-5 m/min, 0.5-20 m/min, 0.5-10 m/min, or 0.5-5 m/min.

In one embodiment, the method of the present disclosure provides thatthe roll-to-roll device is a micro gravure device, wherein the web speedis no more than 5 m/min, 4 m/min, 3 m/min, 2 m/min, or 1.5 m/min. In oneaspect, the web speed is about 0.1-5 m/min, 0.1-2.5 m/min, m/min, 1.0-5m/min, 1.0-2.5 m/min, or 1.0-1.5 m/min.

In one embodiment, the method of the present disclosure provides thatthe roll-to-roll device is a micro gravure device with a speed ratio(web speed/roll speed) of 0.1-3, 0.1-2, 0.1-1.5, 0.3-2, or 0.3-1.5.

In one embodiment, the method of the present disclosure provides thatthe roll-to-roll micro gravure fabrication processes is performed with avariable substrate speed 0.1-50 m/min, m/min, 0.1-10 m/min, 0.2-50m/min, 0.2-25 m/min, 0.2-10 m/min, 0.2-8 m/min.

In one embodiment, the method of the present disclosure provides thatthe aqueous suspension comprises 6-12 wt. % of CNC and 88-94 wt. % ofwater.

In one aspect, the aqueous suspension comprises substantially only CNCand water. In one aspect, the aqueous suspension may comprise CNC,water, and one additional organic polymeric material that has reasonablewater solubility and can form a composite mixture with CNC in aqueoussolution. In one aspect, the organic polymeric material may be but isnot limited to poly(vinyl alcohol) (PVA), starch, polyethylene glycol(PEG), poly(ethyleneoxide) (PEO), poly(3-hydroxybutyrate) (PHB), watersoluble ethylene-vinylalcohol, cellulose derivatives such ascarboxymethylcellulose and hydroxyethylcellulose, natural gums such asxanthan, guar, or Arabic gum, water soluble proteins such as whey orzein, or any derivative/combination thereof.

In one aspect, the aqueous suspension may comprise CNC, water, and oneadditional water dispersible inorganic polymeric material that may beclays such as but is not limited to Montmorrilonite, Laponite, Cloisite,or any combination thereof.

In one aspect, the aqueous suspension may further comprise a plasticizerto make the film more pliable. The plasticizer may be but is not limitedto sorbitol, citric acid or glycerol. Comparing to the pure CNC coating,the coating with CNC and a plasticizer improved the CO₂ barriercapability by 25-50%.

In one aspect, the ratio of the weight of CNC to the weight of awater-soluble polymer or water dispersible inorganic polymeric materialsuch as PVA is at least more than 1.0. In one aspect, the ratio of CNCto the water soluble polymer is about 2:1, 3:1, 4: 1, 5:1, 6:1, 9:1,8:1, 9:1, 20:1, 25:1, 50:1, 100:1. When the water soluble polymer isPVA, the preferred CNC/PVA ratio is about 7:3.

In one aspect, a preferred drying temperature range is between 65-85° C.In one aspect, a preferred drying temperature range is between 75-85° C.

In one embodiment, the present disclosure provides a CNC-coated flexiblesubstrate wherein the CNC-coated flexible substrate comprises ananisotropic CNC coating with an order parameter of at least 0.2. In oneaspect, the anisotropic CNC coating has an order parameter of at least0.3, 0.4, or 0.5. In one aspect, the anisotropic CNC coating has anorder parameter of about 0.2-0.95, 0.2-0.9. 0.2-0.8, 0.2-0.7, 0.3-1.0,0.3-0.95, 0.3-0.9, 0.3-0.8, 0.3-0.7, 0.4-1.0, 0.4-0.4-0.8, or 0.4-0.7.

In one embodiment, the CNC-coated flexible substrate may comprise a CNCcoating of more than one CNC layers. In one aspect, the CNC-coatedflexible substrate comprises 2, 3, 4, 5, 6, 7, 8, 9, or more CNC layers.In one aspect, the CNC-coated flexible substrate comprises 1-100, 1-90,1-80, 1-70, 1-60, 1-50, 1-40, 1-30, 1-20, 1-10, or 1-5 CNC layers.

In one embodiment, the CNC-coated flexible substrate may comprise one ormore coatings with a CNC coating thickness between 100 nm-100 μm, 100nm-50 μm, 100 nm-30 μm, 100 nm-20 μm, 100 nm-10 μm, 500 nm-100 μm, 500nm-50 μm, 500 nm-40 μm, 500 nm-30 μm, 500 nm-20 μm, 500 nm-10 μm, 1μm-100 μm, 1 μm-50 μm, 1 μm-40 μm, 1 μm-30 μm, 1 μm-20 μm, 1 μm-10 μm.

In one embodiment, the flexible substrate may be any plastic likematerial such as but is not limited to polyethylene (PE), polypropylene(PP), polylactic acid, or even paper sheet. In one aspect, the flexiblesubstrate may be any flexible polyester. In one aspect, the flexiblesubstrate may be any flexible polyester such as but is not limited topolyhydroxyalkanoates such as poly-3-hydroxybutyrate,polyhydroxyvalerate, polyhydroxyhexanoate; polylactic acid polyesters;polybutylene succinate, polycaprolactone, starch and starch derivatives,cellulose esters such as cellulose acetate and nitrocellulose andderivatives thereof (such as celluloid), or polyethylene terephthalate(PET). In one aspect, the flexible substrate is polyethyleneterephthalate (PET).

In one embodiment, the CNC-coated flexible substrate comprises asubstantially transparent CNC coating. In one aspect, the substantiallytransparent CNC coating does not decrease the transparency of theuncoated flexible substrate by 30%, by 20%, by 10%, by 5%, or by 2.5%.

In one embodiment, the method of the continuous roll-to-roll coating ofa flexible substrate may prepare CNC-coated flexible substrate with alength of at least 1 meter (m), 10 m, 100 m, 1000 m, 10,000 m, or100,000 m. In one aspect, the length is about 1-100,000 m, 1-10,000 m,1-1000 m, 1-100 m, or 1-10 m.

The following experiments and/or examples are for illustration purposefor the present disclosure.

EXAMPLE 1 CNC Only Coating Materials and Ink Formulation

Flexible polyester films (MELINEX® 462 2 mil) were purchased from TEKRA(New Berlin, WI, USA) and used as a substrate. The substrate film, 1000m in length and 15.24 cm in width, was installed on the roll-to-rollcoating system. Never-dried, pristine CNC (12.2 wt. %, batchno-2015-FPL-071CNC) aqueous suspension purchased from the University ofMaine (Orono, ME, USA) and manufactured by the USDA ForestService-Forest Products Laboratory (FPL) (Madison, WI, USA), was used asink in this investigation. The stock CNC aqueous solution was dilutedwith nano-pure water to a final concentration of 6 wt. %, 9 wt. %, and12 wt. % CNC. Solutions were ultra-sonicated for 10 mins to disperseCNCs and homogenize solutions. Sonication introduced fluidity intosolutions by destroying its gel structure and enabling their use as anactive ink system without any further formulation. The rheologicalmeasurements of the CNC suspension were performed on a shear controlledrotational rheometer (Malvern Bohlin Gemini HR Nano) equipped with a cupand bob fixture. The steady shear viscosity was measured for 100um gapdistance and the shear rate range was 0.01-500 s⁻¹. All measurementswere performed at room temperature.

Substrate and Ink Compatibility

Substrate-ink compatibility is the most important parameter for anyprinting or coating technique. In general, the surface energy of thesubstrate must be higher than the surface tension of the ink system.Flexible polyester films have lower surface energy (45 dynes/cm)compared to CNC dispersion surface tension (60-65 Dynes/cm) (Gardner etal. 2008); hence, surface treatment is essential to overcome de-wettingissue. We utilized a high-speed, high-power corona treater system (QCelectronics, Inc.) for substrate processing, which introduced a surfacemodification to increase the wettability of the substrate roll.Treatment speed rate as well as power supply dominated the overallsurface modification. We observed that a substrate treatment with a 0.5KW power supply and 2 m/min speed rate can produce 60-65 dynes/cmsurface energy on the film, which is compatible with an aqueous CNCsuspension.

Film Fabrication

A Mirwec Mini-Labo Deluxe™ system with a tri-helical (mesh R30)microgravure system has been utilized for our gravure coating. Thegravure coating includes a gravure roller, ink bath, flexible substrate,and drying chamber. A doctor blade was placed over the gravure cylinderto remove the excess ink and maintain a uniformly solution thickness.The nip distance was minimized between the gravure and the substrate toachieve complete liquid transfer. Fabrication processes were performedwith a variable substrate speed (0.3-6 m/min), and gravure speed (4rpm-70 rpm), which controlled the overall coating thickness and quality.After a successful liquid transfer from the ink bath to the flexiblesubstrate, the wet-coated region passed through an inline drying unit.The temperature of the heating chamber was 80° C. for the entire gravurecoating process, which allows for complete drying of the coating. Afterdrying, the transparent CNC-coated substrate was collected on therewinder.

Film Characterization

A Carl Zeiss (Axio observer A1) inverted light microscope was used intransmission mode for thickness measurements. Both 5× and 10×magnification objectives were used for film thickness characterization.A cut sample was placed perpendicularly between the objective and thestage and film thickness was measured. Polarized light opticalmicroscopy was used to image coatings with samples at 45° and 90° withrespect to the plane of polarized light.

The transparency of neat PET and CNC-coated PET films were measured witha conventional UV-Vis spectrophotometer (Spectramax Plus 384, Moleculardevices Corp., Sunnyvale, CA). The transmittance data for each film weremeasured across 400-750 mn wavelength with air as the background.

Anisotropy Measurement

A conventional UV-vis spectrophotometer (Spectramax Plus 384, Moleculardevices Corp., Sunnyvale, CA) was used for the characterization of theCNC alignment. A similar method was reported by Chowdhury et al. SeeChowdhury et al., Improved order parameter (alignment) determination incellulose nanocrystal (CNC) films by a simple optical birefringencemethod Cellulose, 2017, 24:1957-1970. Briefly, a sample was delaminatedfrom the substrate and placed between a cross polarizer and thetransmitted light intensity was measured at 45° and 90° configurations.The transmittance data was recorded from 400 nm to 750 nm wavelength.The following equations were used for the order parameter, Scalculation:

$\begin{matrix}{{I = {I_{0}\sin^{2}2\theta{\sin^{2}\left( \frac{{\pi\Delta}{nd}}{\lambda} \right)}}}{So}{\frac{I_{45}}{I_{90}} = \frac{I_{0}{\sin^{2}\left( {2*45} \right)}{\sin^{2}\left( \frac{{\pi\Delta}{nd}}{\lambda} \right)}}{I_{0}{\sin^{2}\left( {2*90} \right)}{\sin^{2}\left( \frac{{\pi\Delta}{nd}}{\lambda} \right)}}}{{Hence},}} & (1) \\{\frac{I_{45}}{I_{90}} = {D^{*} = {{D \cdot g} = \frac{\left( {{2S} + 1} \right)}{\left( {1 - S} \right)}}}} & (2)\end{matrix}$

Here, I₀, θ, Δn, d, λ, I, g and D represent the amplitude of theincident light, the angle of the material between the cross-polarizer,refractive index difference, film thickness, wavelength, transmittedlight intensity, correction factor and dichroic ratio, respectively. Theparallel and perpendicular refractive indices are relatively low forcellulose; therefore, we adopt g=1 for our calculation. The orderparameter for any material is between 0 and 1, where S=0 is defined asan entirely random/isotropic configuration and S=1 is for a perfectanisotropic arrangement.

Surface Morphology

A Dimension 5000 Atomic force microscope (AFM) in contact mode was usedto examine surfaces of organic coatings on the polyester substrate.Samples were prepared by cutting a section of the film from the middleof the coated section and adhering the strip to a metal puck. Twosamples were selected for each condition (6 wt. %, 9 wt. %, and 12 wt.%) and three sections on each sample imaged. Films were fabricated underthe same conditions at a speed ratio of 1. Films for AFM imaging showedno visual signs of defects.

Adhesion Test

CNC coating adhesion to the flexible substrate was studied with across-hatch adhesion test method (ASTM-D3359). Briefly, a 5×5 cmspecimen was adhered to a glass plate with adhesive spray and leftuntouched for 30 mins for complete drying. With a sharp blade, 11parallel and 11 perpendicular cuts were made on the sample with respectto the center of the film, where each cut was 1 mm apart. A pressuresensitive tape was placed over the film for 5 mins and removed from thesurface at a 180° angle. Both the flexible substrate and CNC coatingwere colorless, so the removal of the CNC coating from the grid area wasinvestigated by polarized light microscopy.

Coating Thickness Control

Parameters such as micro-gravure speed, substrate speed, speed ratio,capillary number, and CNC concentration collectively control the liquidtransfer rate of this fabrication process. A thick liquid layer is thefirst requirement for maximum liquid transfer from the gravure to thesubstrate. The liquid thickness solely depends on a stable andcontinuous liquid bridge formation between the gravure and substrate.However, stretching and shear forces also exist in this liquid bridgewhich can destabilize it; these forces can be controlled by gravureroller and web speeds. The data regarding the coating thicknessdependence on experimental parameters is presented in the tables 1-3 foreach CNC concentration examined.

TABLE 1 6 wt. % CNC suspension speed variables, capillary number, andthickness Uweb, Uroll, m/min m/min Speed ratio Ca Thickness, μm 0.5041.000 0.504 7.8154e−3 2.06 ± .2 0.693 1.000 0.693 7.0277e−3 2.08 ± .481.008 1.000 1.008 6.2400e−3 2.45 ± .21 2.016 1.000 2.016 5.8385e−3 2.42± .54 4.095 1.000 4.095 6.5577e−3  3.1 ± .31 0.630 1.250 0.504 8.2385e−3 2.1 ± .35 0.630 0.960 0.656 6.9308e−3  2.1 ± .32 0.630 0.630 1.0005.6231e−3  2.8 ± .21 0.630 0.450 1.400 4.8385e−3  2.9 ± .32 0.630 0.3102.032 4.0538e−3  3.1 ± .15 1.033 1.010 1.023 6.3702e−3  2.7 ± .44 0.8820.880 1.002 5.9400e−3  2.5 ± .38 0.762 0.760 1.003 5.7012e−3 2.09 ± .310.630 0.630 1.000 5.6231e−3 2.05 ± .24 0.504 0.500 1.008 4.7938e−3  1.9± .41 2.016 1.010 2.000 6.1498e−3 3.39 ± 0.30 1.764 0.880 2.0057.9920e−3 3.25 ± 0.58 1.512 0.756 2.000 6.2954e−3 2.94 ± 0.42 1.2980.650 1.997 5.4718e−3 2.88 ± 0.50 1.021 0.510 2.001 4.8096e−3 2.75 ±0.29

TABLE 2 9 wt. % CNC suspension speed variables, capillary number, andthickness Uroll, m/min Uweb, m/min Speed ratio Ca Thickness, μm 0.5041.000 0.504 0.0313 2.86 ± 0.32 0.693 1.000 0.693 0.0307 3.20 ± 0.481.008 1.000 1.008 0.0364 3.60 ± 0.31 2.016 1.000 2.016 0.0311 3.80 ±0.52 4.095 1.000 4.095 0.0525 4.10 ± 0.48 0.630 1.230 0.512 0.0339 2.60± 0.31 0.630 0.920 0.685 0.0285 2.80 ± 0.32 0.630 0.640 0.984 0.02323.50 ± 0.40 0.630 0.460 1.370 0.0199 4.10 ± 0.41 0.630 0.350 1.8000.0167 4.20 ± 0.43 1.033 1.010 1.023 0.0337 3.59 ± 0.54 0.882 0.8801.002 0.0320 3.70 ± 0.54 0.762 0.760 1.003 0.0285 3.55 ± 0.50 0.6300.630 1.000 0.0232 3.01 ± 0.54 0.504 0.500 1.008 0.0227 2.90 ± 0.432.016 1.008 2.000 0.0311 4.66 ± 0.46 1.764 0.880 2.005 0.0342  4.4 ±0.63 1.512 0.756 2.000 0.0286 4.59 ± 0.62 1.298 0.650 1.997 0.0251 4.11± 0.72 1.021 0.510 2.001 0.0231 4.37 ± 0.37

TABLE 3 12 wt. % CNC suspension speed variables, capillary number, andthickness Uroll, m/min Uweb, m/min Speed ratio Ca Thickness, μm 0.5041.000 0.504 0.0625 4.38 ± 0.29 0.693 1.000 0.693 0.0615  4.5 ± 0.431.008 1.000 1.008 0.0572 4.72 ± 0.48 2.016 1.000 2.016 0.0584 4.94 ±0.27 4.095 1.000 4.095 0.0787 6.27 ± 0.52 0.630 1.260 0.500 0.0727 3.72± 0.19 0.630 0.950 0.663 0.0612 4.16 ± 0.32 0.630 0.630 1.000 0.04964.94 ± 0.29 0.630 0.470 1.340 0.0427 5.08 ± 0.41 0.630 0.320 1.9690.0358 5.41 ± 0.52 1.033 1.010 1.023 0.0579 4.38 ± 0.26 0.882 0.8801.002 0.0548 4.22 ± 0.19 0.762 0.760 1.003 0.0532 3.94 ± 0.24 0.6300.630 1.000 0.0496 3.72 ± 0.26 0.504 0.500 1.008 0.0404 3.72 ± 0.212.016 1.008 2.000 0.0584 5.05 ± 0.47 1.764 0.880 2.005 0.0546 4.94 ±0.40 1.512 0.756 2.000 0.0492 4.94 ± 0.38 1.298 0.650 1.997 0.0487 5.08± 0.55 1.021 0.510 2.001 0.0423 5.20 ± 0.42

At a constant web speed, coating thickness increases with increasingroller speed and increasing CNC concentration. By doubling the CNCconcentration from 6 wt. % to 12 wt. % the coating thicknessapproximately doubled for roller speeds less than 4 m/min. Similarly,the 9 wt. % is approximately 1.5 times greater than the 6 wt. %coatings. For these conditions, solid loading of the CNC nanoparticlesin the ink primarily appeared to control coating thickness. However, theviscosity and therefore capillary number, which increases monotonicallywith viscosity, are also expected to contribute to coating thickness asthese parameters should theoretically improve the liquid transfer rate.At higher roller speeds, the capillary number for 12 wt. % is 12 timeshigher than 6 wt. % CNC suspensions, and coating thicknesses greaterthan twice the 6 wt. % coatings were achieved. At the higher rollerspeeds, the increase in thickness may be owing to the significantincrease in capillary number and hence better liquid transfer rate fromthe roller to the substrate. While the 9 wt. % concentration followed asimilar liquid transfer behavior to the 12 wt. % CNC suspension, the 6wt. % CNC displayed the lowest capillary numbers for this system; hence,the liquid ink exhibits Newtonian behavior and follows a symmetricalliquid bridge breakup. Consequently, a Newtonian liquid cannot de-wetcompletely from the gravure cell cavity resulting in low ink transferand therefore, low coating thickness.

On the other hand, constant roller rotation with a variable web speeddemonstrates the opposite behavior due to the reduced liquid transferrate. At a low web speed, the roller can rotate more than one time overthe web surface resulting in the maximum coating thickness at low webspeeds. However, liquid transfer rate reduces gradually with increasingweb speed, therefore lowering the coating thickness. In general, thedirection of the stretching force should be along the web direction, andthe inertia force of the liquid will be along the roller direction. Bothare altered by changing the web speed. At higher web speed, the inertiaforce dominates the stretching force, which facilitates a lower liquidtransfer rate to the web surface. For variable web speed, increasingconcentration from 6 wt. % to 9 wt. % and 12 wt. % did not producecoatings with respective thickness 1.5 to 2 times greater, therefore,viscous and inertia forces dominated solids loading.

The speed ratio (defined as web speed divided by roll speed) is acritical parameter dependent upon on the roller and substrate speeds,where the capillary number of the liquid suspension controls the liquidtransfer rate for the fabrication process. The thickness of theresultant films, and therefore liquid transfer rate from the ink bath tothe substrate, increased by a small amount with increasing speed ratiofor all CNC concentrations. The liquid transfer rate also depends on theviscous force and inertia force where viscous forces must dominate theinertia force to increase the coating thickness. At a lower speed ratio(0-0.5), the viscosity of the 12 wt. % CNC suspension is very highcompared to the other concentrations. So, liquid transfer rate must behigher for the 12 wt. % suspension based on capillary number alone.Similarly, 9 wt. % has a higher viscous force compared to the 6 wt. %suspension and resulted in a higher transfer rate and overall highercoating thickness than the lowest concentration. Due to the extremelyshear thinning nature of CNC dispersions, increasing shear rate(governed by increasing gravure rotation) will reduce ink viscositysignificantly, which will reduce the liquid transfer rate at the higherspeed ratio region. At this reduced viscosity, all CNC concentrationsbehave as Newtonian fluids in which symmetrical liquid bridge formationis the liquid transfer mechanism. Moreover, in the high-speed ratioregion, the capillary number is also reduced because of the reduction insuspension viscosity at high shear rates. The reduction of the capillarynumber and symmetrical liquid bridge formation are the primary reasonsfor steady state liquid transfer at high-speed ratios where the samplethickness begins to asymptote for all suspensions.

The highest concentration suspension (12 wt. %) shows a wider region ofcoating thickness with increased gravure rotation compared to the 6 wt.% and 9 wt. % counterparts. Speed ratios from 2 to 3.2 show a highlydefected coating and so are of limited utility. However, defect-freecoatings with reduced thickness were observed at lower speed ratios andhigher web speeds. The 9 wt. % CNC suspension shows a much smallerregion for the thickest coatings with slow gravure rotation. It appearsthat the maximum liquid transfer rate can be attributed to higherviscosity ink systems.

Effect of Film Thickness on Coating Adhesion

Samples of various coating thicknesses were subjected to crosshatchadhesion tests to measure coating adhesion. Adhesive bonding within thecoating and at the coating-substrate interface varies with the coatingthickness. Since visual inspection of de-bonding on transparent coatedPET film was challenging; polarized light was used to quantify theadhesion loss of CNC coatings. Samples were illuminated between crossedpolarizers, and the damaged area was identified based on the colorcontrast. The quantification of the adhesion loss via coatingperformance was categorized on a scale of 0-100%. Based on the ASTMD3359, samples were classified into 6 different categories as follows:5B (0%), 4B (less than 5%), 3B (5-15%), 2B (15-35%), 1B (35-65%) and 0B(>65%) where 5B is considered optimal with 0% area removed from thesubstrate.

CNC coatings exhibit high peel strength from the PET substrate.Substrate-coating interaction plays a significant role in the peelingstrength and is determined by the surface energy and surface tension ofthe substrate and ink (CNC dispersion in water), respectively. Hence, ahighly wettable coating shows very strong adhesion. Corona or plasmatreated PET substrates, which have a higher surface energy and thereforeare more wettable, show much higher adhesive strength compared tountreated PET substrates. A similar behavior was also observed for thecrosshatch experiments here since a corona treatment was applied to thePET substrate.

Coating adhesion displays a strong dependence on coating thicknessdespite lowering the surface energy difference at the interface throughsurface treatment. The total coating thickness from 6 wt. % CNCsuspension was between 2-3 μm, and no delamination was observed for thiscoating thickness range. Increasing the CNC concentration to 9 wt. % CNCsuspension increases the final coating thickness to 3.5-4.5 μm andcoincides with the onset of delamination in the adhesion test of coatedfilms and therefore a decrease in coating performance. Furthermore, CNCcoatings produced with the 12 wt. % CNC suspension (4.5-6 μm totalthickness) demonstrate the lowest adhesive rating compared to 6 wt. %and 9 wt. % suspension. The cohesive strength of the bulk CNC coating,and the adhesive strength between the substrate-coating interfaces canexplain the above observation. As a rule of thumb, a reduced coatingthickness along can improve the coating performance. As the finalcoating thickness depends on the initial CNC concentration, thinnestcoatings produced by the 6 wt. % suspension, possessed the maximum peelstrength. Theoretically, the cohesive strength should not changesignificantly for any thickness difference; moreover, the adhesivestrength at the interface is independent of the coating thickness. Itwas expected the adhesive rating to be similar for all CNCconcentrations, but it was unexpectedly observed delamination for 9 wt.% and 12 wt. % CNC suspension-based coating. Fabrication defects presenton the surface and in bulk coating regions, which strongly depend onfilm thickness, may initiate cracks in the coating that result indelamination. As well, there may be residual stress in the films owingto their rapid drying and low solids gel point.

The mechanism of defect formation in roll-to-roll system stronglydepends on the viscosity and capillary number of the ink system. Here,high viscosity liquids (9wt % and 12 wt. %) will have higher capillarynumbers. Higher capillary number fluids can entrap air bubbles on thegravure, which can lead to different coating defects. Moreover,high-speed gravure rotation of a viscous ink system can also produce airbubbles in the ink bath which in turn may produce defects in thickercoating samples not present in thinner coatings. The presence of avolume defect like cavitation can reduce the adhesive bond between theinterfaces and therefore reduce the adhesive strength which results indelamination or de-bonding at higher coating thicknesses.

Effect of CNC Concentration on Surface Morphology and OpticalTransparency

Surface morphology varied greatly with CNC concentration. As CNCconcentration was increased, the surface roughness was seen to increase.Both the 9 wt. % and 12 wt. % surface morphologies were considerablyrougher compared to the 6 wt. % suspension, which is reflected in R_(q),the root mean squared height deviation, of Table 4. The presence ofgroove or current-like morphologies in the 9wt % indicate a strongdirectional dependence of the surface morphology. The direction of thegrooves approximately coincides with the 0-20° orientation introduced bythe gravure tilt along the direction of shear. For highly viscoussuspensions like the 9 wt. % and 12 wt. % the relaxation time is longerand so solutions will retain more orientation during processing, as seenwith the order parameter in the subsequent section, in the rollingdirection. The scale and frequency of this morphology are both too smallto be gravure patterning. The unique appearance may be a result of localde-wetting of the suspension from the surface similar to microscaledefects seen at higher speed ratios. While the 12 wt. % is still rough,it did not exhibit a strong direction dependence did in the areasexamined. It did have several pockets, which may be the result ofcavitation. Cavitation can occur when air becomes entrapped in a viscoussolution and is transferred to the surface. The 6 wt. % exhibited smallvariation in sample roughness, see Table 4. This is not surprising asthe suspension behaved as a Newtonian fluid and has a fast relaxationtime compared to the 9 wt. % and 12 wt. %. In particular, the maximumvertical height difference, Rmax, and Rq were proximately twice as largefor the 9 wt. % and 12 wt. % CNC coatings compared to the 6 wt. %.Increased surface roughness creates a stronger adhesive bond between thecoating and the tape in the peel test by strengthening the mechanicalbond between the surface and tape. Increased surface roughness can beproblematic as defects and uneven surfaces are sites for failure. Theincreased roughness of the higher suspensions may be one reason the 9wt. % and 12 wt. % adhesion performance was not as high compared to the6 wt. %.

TABLE 4 Roughness measurements as a function of CNC content CNC Content(wt. %) R_(q) (nm) R_(max) (nm) 12 12.40 ±/− 2.11 98.40 ±/− 25.84  913.00 ±/− 2.07 104.9 ±/− 12.83  6  5.87 ±/− 0.38 53.03 ±/− 3.52 R_(q):Root mean squared deviation in height; R_(max) = Maximum vertical heightdifference.

Surface morphology is indicative of the liquid transfer efficiency aswell. A liquid bridge exists between the gravure and the substrate inwhich ink is transferred. Across this liquid bridge there can be asurface tension gradient which will help drive liquid transfer. If thesurface tension of the liquid is minimized by wetting the substrate, itwill do so. A reasonable expectation is that the surface morphologywould be smooth and flat if surface tension favors the transfer of theink to the substrate. However, viscous forces will be competing againstsurface tension. As the viscosity of the liquid increases the capillarynumber increases. For high viscosity, high Ca, the viscous forces willdominate surface tension effects in flow behavior and the surface maystill appear rough despite liquid transfer being a favorable process. Inthe present study, the capillary number is increasing with increasingCNC concentration which may explain why surface roughness is increasingsignificantly despite favorable wetting conditions after coronatreatment and layer-by-layer coating.

Exceptional optical transparency was obtained for CNC-coated PET films.Flexible PET demonstrated 89-92% transparency for a wavelength rangefrom 400-750 nm, while CNC coating transparency degraded only 2%compared to the bare substrate. Of the CNC coatings, the 12 wt. %transparency was 1.5% lower than the 6 wt. % and 9 wt. %. The presenceof defects (due to a higher capillary number and gravure rotation) orhigher surface roughness in 12 wt. % CNC coating may be responsible forthe slight decrease. The diffuse reflection of incident light ispromoted by surface roughness and may cause light scattering, reducingthe ultimate transparency of the 12 wt. % film. Moreover, edges ofdefects can cause diffraction of the incident light and therefore also areduction of transparency.

Gravure Rotational Effect on Anisotropy

The crystalline domain arrangement (anisotropy, in this case) is largelydetermined by the combined action of the shear rate and domain retentiontime, which depends on the gravure rotation and the rheology of the CNCsolution. To calculate Herman's order parameter, S, light transmittanceof free-standing CNC films was measured. Films were created by varyinggravure rotation (which determines shear rate) and CNC concentration at6 wt. %, 9 wt. %, and 12 wt. %. A minimum of four layers were used toachieve coatings of 16-20 μm in thickness which could be delaminatedfrom the PET substrate for light transmittance measurements. Thetransmitted light intensity profile for 45° and 90° configurations weremeasured for the determination of the linear dichroic ratio, D, that canbe related to the order parameter, S, for each CNC film through equation2. The maximum transmitted light intensity increases with increasing CNCink concentration for the 45° configuration while the 90° configurationis generally decreasing (Table 5). D, calculated as the ratio of themaximum transmitted intensity between the 45° and 90° configurations,increases from 1.79 for the 6 wt. % up to 4 for the 12 wt. % and fixedshear rate. This suggests that preferential alignment of the crystallinedomains along the shear direction is improved by increasing CNCconcentration. D is maximized for the 12 wt. % samples which impliesthat the difference in refractive index for longitudinal and transversedirections is also highest for the 12 wt. % compared to the otherconcentrations.

TABLE 5 Dichroic ration from maximum transmittance intensity fordifferent specimen and resulted Herman order parameter (S) TransmittanceTransmittance light intensity light intensity CNC for 45° for 90°Dichroic Order concentration configuration configuration ratio parameter 6 wt. % 21.3 11.9 1.79 0.21 22.9 13.5 1.70 0.19 27.4 14.1 1.94 0.2423.3 12.32 1.89 0.23 30.1 15.1 2.05 0.26  9 wt. % 34.2 11.4 3.00 0.4038.6 11.4 3.40 0.45 40.1 10.6 3.80 0.49 31.4  8.5 3.70 0.48 36.7 10.23.60 0.47 12 wt. % 40.6 10.2 4.00 0.50 37.9  8.2 4.60 0.55 42.6  6.56.50 0.65 44.9  8.2 5.50 0.60 38.6  8.4 4.60 0.55

An increasing order parameter as a function of gravure rotation/shearrate is observed and likely due to shear-induced alignment of the CNCsduring the gravure process. At a low shear rate of 80 s⁻¹, 12 wt. %films show an order parameter of ˜0.5. As anticipated, an increase inshear rate enhances domain alignment along the shear direction, but onlyby a modest amount. A maximum order parameter of 0.65 is achieved at 170s⁻¹ shear rate for the 12 wt. %. However, a negligible reduction in theanisotropy of the CNC coating is observed at the higher gravurerotation, and the order parameter is almost constant at the elevatedshear rate region. On the other hand, 9 wt. % samples show lessanisotropy at any gravure rotation and increasing the shear rate had asmaller effect on the domain alignment. No anisotropic properties wereobserved in 6 wt. % CNC samples. Regardless, in at all concentrations,the natural chiral nematic domain structure was sufficiently disturbedto allow for high transparency.

The crystalline domain of the anisotropic CNC coating must align itscrystal position along the shear direction during coating and therelaxation time for the CNC domain movement should be lengthy. Thus, theinitial CNC concentration contributes to alignment. A low concentrationCNC suspension behaves like a Newtonian fluid and the domains show afast relaxation time after applying shear. With enough time domains showa highly isotropic configuration. However, the Newtonian nature may alsolimit orientation in the first place. It is seen in 6 wt. % CNCsuspension that very little alignment remains even after only 5 s ofrelaxation. However, at 9 wt. %, it known that CNC domains can bealigned along the shear direction. However, in previous work, theanisotropy was completely destroyed after 5-10 mins despite havingachieved 50% alignment for a 9 wt. % concentration. See Haywood et al.,Effects of liquid crystalline and shear alignment on the opticalproperties of cellulose nanocrystal films Cellulose, 2017 24:705-716. Inthe present disclosure, 1 m/min web speed was used. And the ink bath todrying unit distance was 50 cm. So the time before drying is 30 secs.Assuming a zero time to dry once it enters the oven, then the longrelaxation time as compared to the dry time would lead to a highalignment retention, so ˜50% anisotropy is logical considering theprevious work getting similar results with simple shear. Similarly, thealignment of the 12 wt. % CNC suspension has a relaxation time of 30mins, but again, is locked into place in 30 s so the even higher orderparameter (S=0.65) is achieved. Little disorientation was observed overthis time scale.

EXAMPLE 2 CNC-PVA Composite Materials and Methods

Poly(vinyl alcohol) 89-98K with 98% hydrolysis were purchased from sigmaAldrich. Never dried, pristine CNC (12.2 wt. % batch no-2015-FPL-71 CNC)aqueous suspension that contains 1% sulfur and a sodium counter ion, waspurchased from University of Maine and manufactured by the USDA-USForest Service-Forest Products Laboratory (FPL) (Madison, WI, USA)(Reiner & Rudie, 2013). All materials were used as active inkingredients without any purification. A stock solution of 12 wt. % PVAsuspension was prepared by adding 36 g of PVA in 300 g water and heatedfor 3 hrs at 120° C. Stock solution of CNC (12% in water) was added tothe PVA solution at a fixed concentration to produce the finalcompositions. The final compositions of different CNC-PVA coating ratiosare shown in Table 6. Before fabrication, all ink suspensions wereultrasonicated (Branson Ultrasonics, Danbury, CT, USA) for 3 mins (at50% amplitude with a 1 s plus and 1 s rest at 60 Hz frequency) todisperse CNCs and homogenized solution. Flexible PET, MELINEX® 462 2 mil(TEKRA) and PLA films (Cargill Dow LLC) were used as substrates for thecoating process.

TABLE 6 Coating formulations for different compositions for CNC-PVA 12wt. 12 wt. Film Film % CNC % PVA thickness thickness suspension,suspension, on on Composition g g PET, μm PLA, μm PVA  0 100 5.8 ± .331.93 ± .09 CNC: PVA (25:75)  25  75 4.6 ± .47  1.8 ± .23 CNC: PVA(50:50)  50  50 4.7 ± .25 1.58 ± .13 CNC: PVA (70:30)  70  30 4.3 ± .151.57 ± .15 CNC: PVA (90:10)  90  10 4.2 ± .21 1.47 ± .15 CNC 100  0.03.8 ± .06 1.23 ± .09

Rheological measurements for CNC-PVA suspensions were performed with aMalvern Bohlin Gemini HR Nano rheometer. A cup and bob fixture with 100μm gap distance was used for each experiment. All measurements wereperformed with a variable shear rate (0.1-500 s⁻¹) at room temperature.

Surface Tension Measurements

Surface tension measurements were obtained at the ink/air interface bypedant drop tensiometry. A Ramé-Hart contact angle goniometer withDROPimage Advanced software (Model 500) was used for thischaracterization. The geometrical profile of the static pedant drop wascompared with the theoretical (Laplace equation) drop profile. Allmeasurements for various ink compositions were performed at roomtemperature and humidity.

Coating Equipment and Conditions

A Mirwec Mibi-Labo Delux™ system with a trihelical (mesh R30 and meshR90) microgravure system was utilized for coating fabrication. Gravureroller, ink batch, doctor blade, flexible substrate and a drying chamberare the major components for this instrument (Figure S1). The gravureroller was placed with an ink bath that continuously transferred inkfrom an ink bath to the substrate. A doctor blade with 50 μm gapdistance was placed over the gravure roller which continuously removedexcess ink from the gravure roller, thus producing uniform coatingthickness. The fabrication process was performed at 1 m/min substratespeed with 30 rpm gravure rotation. Two temperatures were used for thisfabrication based on the heat deflection temperature of the substrate.Coating on the substrates was performed at 80° C. and 45° C. for PET andPLA, respectively.

Both PET and PLA substrates have low surface energy compared to thesurface tension of the ink system. As a result, a surface treatment ofthe substrate was essential to overcome de-wetting (caused byink/substrate incompatibility). A high-power corona discharge system (QCelectronics, Inc.) was used for substrate processing. A 0.5 KW powersupply with 0.5 m/min substrate speed rate was sufficient to increasethe substrate surface energy (60-65 dyne/cm) and enhance ink/substratecompatibility.

Film Characterization

A conventional UV-Vis spectrophotometer (Spectramax Plus 384, Moleculardevices Corp., 133 Sunnyvale, CA) was used for the transparencymeasurements. All measurements were performed between 350-750 nmwavelength ranges with air as the background. Coating thickness of thesamples were observed by a scanning electron microscopy (SEM, Pro X,Phenom). Before imaging, an electrically conductive thin layer of goldcoating was sputtered on the specimen.

A conventional UV-Vis spectrophotometer (Spectramax Plus 384, Moleculardevices Corp., 133 Sunnyvale, CA) was used for the transparencymeasurements. All measurements were performed between 350-750 nmwavelength ranges with air as the background. Coating thickness of thesamples were observed by a scanning electron microscopy (SEM, Pro X,Phenom). Before imaging, an electrically conductive thin layer of goldcoating was sputtered on the specimen.

Excellent optical transparency was observed for CNC-PVA coatings on theflexible PET substrate. The uncoated PET substrate showed 89-92% opticaltransparency for a wavelength range of 350-750 nm. There was nosignificant transmittance reduction for any CNC-PVA composition relativeto the uncoated PET film. Therefore, different coating compositions haveminimal light scattering and reflection. This high transparency isimportant in commercial applications particularly in food packagingwhere clear display of the food product is required.

Effect of CNC Loading on Coating Anisotropy

For a constant shear rate (approximately 300 s⁻¹), it is observed fromFIG. 1 that CNC loading controls the overall anisotropy of the CNC — PVAcoatings. At 0-50% CNC loading in the dry film, the ink system mostlydisplayed an isotropic arrangement. However, there was a sudden increasein anisotropy up to a maximum order parameter of 0.78 at 70% CNCloading. The anisotropy was steady up to a 90% CNC loading. Finally, theorder parameter reduced to 0.65 for pure CNC coating.

In the aqueous system, due to its long chains, PVA has a swollen gelstructure with many entanglements. Hence, shear alignment must bedifficult for the PVA solution. Therefore, the CNC is the key factor forthe coating anisotropy, whose alignment is determined by the combinedeffect of shear rate and crystalline domains retention time. The shearrate depends on the gravure rotation. However, a constant gravurerotation was used for this fabrication, so the overall coatinganisotropy should mainly depend on the CNC content and the domainrelaxation time. The relaxation time for CNC movement is determined bythe viscosity of the ink system. More viscous ink systems are expectedto exhibit higher relaxation times as compared to less viscous ones. Asviscosity is decreased with increased CNC percentage (FIG. 2 ), it isexpected that higher CNC loading ink should have a faster relaxationtime thus corresponding with a lower order parameter. However, highlyanisotropic coatings were observed at higher CNC loadings even thoughink viscosity decreased compared to lower CNC percentage loading. ForCNC loading less than 50 wt. %, the applied shear rate (300 s⁻¹) may notbe sufficient to produce an anisotropic coating. This could be due tothe entrapment of the CNC domains in the globular structure of the PVAchain that hinders the alignment of CNC domains along the sheardirection. To validate this assessment, real time birefringencemeasurements were made for the various CNC-PVA compositions asillustrated in FIG. 3 .

Real-time particle movement was investigated along the shear directionfor different PVA-CNC compositions. It should be noted that thisexperiment has been performed in an aqueous system with a constant shearrate (300 s⁻¹). Little birefringence was observed for less than 50 wt. %CNC loading. This is a clear indication of a mostly isotropicarrangement. As CNC percentage loading was increased to 70 wt. %, thepresence of birefringence was observed with intense coloration. Thus, acertain threshold CNC percentage loading is required to exhibitlarge-scale birefringence. The maximum color intensity of the 70% and90% CNC composition confirmed maximum anisotropy, which is consistentwith the order parameter study shown earlier. The pure CNC coating alsoshowed birefringence, but color intensity reduced over the observationtime which suggested faster domain relaxation. The experiment wasperformed under open atmosphere so three possibilities arise. In thefirst scenario, faster water evaporation may lead to rapid thicknessreduction of the CNC-PVA solution, which resulted in the fading of thecolor over time. The second reason could be the effect of Brownianmotion or rapid chain relaxation leading to thickness variation thatproduced the color change overtime. The change in momentum of thecrystalline domains or polymer chains could be another reason for colorvariation. As a rapid shear force is applied on the CNC-PVA solution,the CNC crystalline domains and the PVA polymer chains could acquireenergy by attaining higher momentum. However, as the source of thisforce is removed, the crystalline domains or polymer chains begin torelax. The CNCs or polymer chains lose their acquired momentum bydissipating the adsorbed energy which is accompanied by the variouscolor changes.

Water Vapor Transmission Rate (WVTR)

Water vapor transmission rate (WVTR) is a fundamental parameter thatdetails the degree to which water vapor (humidity) can transport througha solid film. A good barrier film/coating for food packagingapplications will have a low WVTR value. The mechanism of water vaportransmission depends on free volume, packing density,crystalline-amorphous ratio and hydrophilic/hydrophobic nature of thepolymeric materials. PET substrates possess excellent water resistancecompared to CNC-PVA composite coatings. Hence; PLA was chosen as it isknown to have a relatively high WVTR that precludes its use in manyapplications. WVTR for different CNC-PVA coatings on the poly (lacticacid) (PLA) substrate has been shown in FIG. 4 . There was a noticeablereduction in the WVTR with increasing CNC loading. The crystallinedomains of CNC act as a physical barrier (due to higher packing density)and creates a longer tortuous path for the permeation of moisturemolecules.

The lowest WVTR was observed at 70% CNC loading as compared to any othercomposition. As indicated earlier, this composition exhibited themaximum order parameter, 0.78, which reduced the free volume of thecoating system. So, the combined effect of reduced free volume andincreased path-length due to the CNC content lowered the WVTR. However,for CNC loading more than 70%, permeability again increased withincreasing CNC loading which clearly contradicts expectation previouslyreported. See Wahyuningsih et al., Utilization of Cellulose fromPineapple Leaf Fibers as Nanofiller in Polyvinyl Alcohol-Based FilmIndonesian Journal of Chemistry 16:181-189. Based on this experimentalresult, two possible reasons can be deduced for the increment of WVTR.There must be some critical PVA percentage loading where PVA chains caneither fill the gap among CNC domains or form a network structure withthe CNCs through intermolecular hydrogen bonding with minimum freevolume. Below this critical PVA percentage loading, there should be somefree volume that enhances WVTR. Another possible reason can beattributed to swelling of the CNC—PVA polymer coating system. Aftermoisture absorption, CNC domains can dislocate from their originalposition which may provide additional free volume by reducing theoverall order parameter.

1. A cellulose nanocrystal-coated flexible substrate comprising aflexible substrate and at least one cellulose nanocrystal layer that isanisotropic and has an order parameter of at least 0.2.
 2. The cellulosenanocrystal-coated flexible substrate of claim 1, wherein the orderparameter of the at least one cellulose nanocrystal layer is 0.2 to0.95.
 3. The cellulose nanocrystal-coated flexible substrate of claim 1,wherein the order parameter of the at least one cellulose nanocrystallayer is 0.6 to 0.95.
 4. The cellulose nanocrystal-coated flexiblesubstrate of claim 1, wherein the thickness of the at least onecellulose nanocrystal layer is 1 μm to 10 μm.
 5. The cellulosenanocrystal-coated flexible substrate of claim 1, wherein the at leastone cellulose nanocrystal layer is a layer of a substantiallytransparent cellulose nanocrystal coating.
 6. A cellulosenanocrystal-coated flexible substrate comprising a cellulosenanocrystal-containing coating on a flexible substrate, wherein thecellulose nanocrystal-containing coating is anisotropic and has an orderparameter of 0.6 to 0.95.
 7. The cellulose nanocrystal-coated flexiblesubstrate of claim 6, therein the cellulose nanocrystal-containingcoating further comprises a water-soluble organic polymeric material ora water dispersible inorganic polymeric material.
 8. The cellulosenanocrystal-coated flexible substrate of claim 6, therein the cellulosenanocrystal-containing coating further comprises a water dispersibleinorganic polymeric material.
 9. The cellulose nanocrystal-coatedflexible substrate of claim 6, therein the cellulosenanocrystal-containing coating further is a water-soluble organicpolymeric material.
 10. The cellulose nanocrystal-coated flexiblesubstrate of claim 9, wherein the water-soluble organic polymericmaterial is poly(vinyl alcohol).
 11. The cellulose nanocrystal-coatedflexible substrate of claim 10, wherein the cellulosenanocrystal-containing coating has an order parameter of 0.7 to 0.95.